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LB agar, also known as lysogeny broth, is a multi-purpose media capable of growing a wide variety of microorganisms. LB agar plates are pale yellow in color as shown in Figure 4.1. LB is classified as a non-selective and non-differential media. LB agar is commonly used for growing E. coli, a microbe often associated with molecular microbiology applications based on its ability to easily produce recombinant proteins.

Trypticase Soy Agar

Trypticase soy agar, also known as TSA or TSAYE, is another multi-purpose media capable of growing a wide variety of microorganisms. TSA plates are virtually identical in color to LB plates. TSA media is non-selective and non-differential and serves as the base when formulating specialized (enriched) medias.

Blood Agar
Blood agar, also known as BAP (blood agar plates), is a derivative of TSA whereby mammalian blood, often from sheep, is added to the plate composition at a concentration range of 5-10%. As such, BAP have a distinct red coloration. BAP is an enriched, non-selective yet differential media. The red blood cells in the media promote the growth of fastidious microorganisms, such as various strains of Streptococcus. BAP is a differential media used to detect hemolytic activity. Hemolysis, the lysis (breakdown) of red blood cells, is classified as alpha, beta, or gamma. When grown on BAP, microbes capable of alpha hemolysis present as greenish-brown color colonies due to the incomplete (partial) lysis of red blood cells. Beta hemolysis is classified as the capacity of a microbe to completely lyse red blood cells. The resulting lysis presents as a distinct zone of clearing around the growing colony. Gamma hemolysis designates the absence or lack of hemolytic activity, and the resulting colonies are often white/tan in color growing on the red background color of the unaffected blood agar plate.

Columbia CNA Agar

Columbia CNA agar is red in color and is an enriched, selective, and differential media. As the media contains the antimicrobial agents colistin and Nalidixic acid, it suppresses the growth of Gram-negative bacteria and is, therefore, used for isolation of Gram-positive microbes. Similar to BAP, the media is also enriched with blood, which allows for differentiation based on hemolytic patterns.

Chocolate Agar
Chocolate agar, also known as CHOC or as chocolate blood agar (CBA), is an enriched, non-selective, non-differential derivative of blood agar plates. Unlike blood agar plates that contain intact red blood cells, chocolate agar plates contain red blood cells that have been lysed by heat. This cooking of the red blood cells gives the media a dark brown chocolaty appearance, hence the name. Note: Actual chocolate is never added to the agar. The lysed red blood cells release various growth factors into the media required for the cultivation of fastidious, pathogenic bacteria. As such, chocolate agar is also considered an enriched media. Examples include the Gram-negative bacteria Haemophilus influenzae and Neisseria meningitidis.

MacConkey Agar

MacConkey agar is pale red in color and is both a selective and differential media. The presence of crystal violet and bile salts in its formulation restricts Gram-positive bacterial growth. As such, only Gram-negative microbes can be grown on MacConkey agar. Further, lactose and the pH indicator neutral red—it is only red when under acidic conditions—are added to differentiate between lactose fermenters (lac+; red colonies) and non-fermenters (lac-; white/tan colonies). The base color of the agar plate also often changes color as the microbes consume the nutrients within the agar. The agar can turn yellow (pH > 8.0) within the surrounding area of a growing non-fermenting microbe. Similarly, the agar may turn pink or even a darker red within the region surrounding a lac+ microbe given its level of acid production during fermentation (pH < 6.8). For example, the Gram-negative/lac+ bacteria Escherichia coli is often used to demonstrate the formation of vibrant pink colonies. Within the clinical laboratory setting, where each patient sample may contain hundreds of species of bacteria, MacConkey agar is used to isolate intestinal pathogenic microbes belonging to the Enterobacteriaceaefamily such as Salmonella and Shigella species.

Sorbitol-MacConkey Agar
Sorbitol-MacConkey agar, also known as SMAC, is a variant of MacConkey agar specifically formulated to detect the presence of the pathogenic strain of Escherichia coli O157:H7. Under standard enteric (gut) conditions, Escherichia coli is able to ferment both lactose and sorbitol. However, E. coli O157:H7 is unable to ferment sorbitol. Therefore, SMAC plates contain sorbitol (instead of lactose) in order to differentiate non-pathogenic strains of Escherichia coli (red colonies; acidic conditions) from pathogenic O157:H7 strains (white colonies; neutral to basic conditions).

Eosin Methylene Blue Agar
Eosin Methylene Blue agar, also known as EMB, is red in color and is classified as both a selective and differential form of media. As the naming suggests, EMB contains eosin and methylene blue that restricts the growth of Gram-positive bacteria. Microorganisms are differentiated based on their ability to ferment the lactose present in the media. Similar to MacConkey agar, non-fermenting (lac-) microbes grow as white/tan colonies, while lactose-fermenting microbes produce dark (purple to black) colonies on the surface of the agar. Most noticeably, if Escherichia coli is grown on EMB agar plates, colonies will have a distinctive metallic green sheen.

Mannitol Salt Agar
Mannitol salt agar, also known as MSA, is red in color and is both a selective and differential media. MSA is selective for Gram-positive bacteria and can differentiate members of the Staphylococci family. MSA contains a high concentration of sodium chloride (~7.5-10%) that prevents the growth of other microorganisms (selectivity) while the presence of an alternate sugar source, mannitol, along with the pH indicator dye phenol red serves to differentiate pathogenic from non-pathogenic strains of Staph. Pathogenic strains, such as Staphylococcus aureus, have the ability to ferment mannitol, thus lowering the pH of the medium and changing the dye color from red (pH >8.2) to yellow (pH < 6.8).